Merge(?): World coordinate system values

[cmaumet]

Following discussion at #52 with @nicholst, @satra and @jbpoline, a first implementation of the WorldCoordinateSystem terms is included in this pull request.

The current hierarchy is as follows:

Definitions must be re-worked. In particular, for each CoordinateSystem, we would need to specify (as suggested by @satra):

1. a set of images
2. a normalization algorithm
3. the previous template

[nicholst]

+1

[cmaumet]

This PR has just been rebased.

Do you think we could merge these changes?

[satra]

@cmaumet - this looks good.

my two worries (not necessary to prevent merging for now) are:

1. can we keep URLs as short as possible? as such could create further hierarchies for the MNI152Nonlinear[a|b|c][Asymmetric|Symmetric]CoordinateSystem and thereby reduce the term name.
2. i think "CustomCoordinateSystem" should be parallel to worldCS. not that it matters in brain imaging now, but you could fix a coordinate system in space, what if they put a portable scanner on the international space station or as part of the one way trip to mars in 2024 :)

btw @khelm do scanners contain information about orientation of the bore and latitude/longitude of the scanner.

[cmaumet]

@satra: thank you for your comments. Regarding your point 2: I am wondering if World is the appropriate prefix...

I liked World because it goes well with the voxelToWorldMapping attribute. In this thread on the nipy mailing list (pointed out earlier by @jbpoline) they used both "world coordinate"/"world space" and "reference space".

Would Reference be clearer than World?

[khelm]

@satra: I will check. I know it's been difficult to get from vendors info on their gradient coordinate system, how they define the patient orientation etc, so I don't know if they put it in DICOM or not. Lat/Long would be a surprise.

[nicholst]

+1 to merge

[cmaumet]

Thank you for the comments! I will now merge those changes. I have created issue #139 to continue the discussion on the naming and hierarchy of those terms.